A land or transition zone survey, such as a seismic survey, sometimes has to cover certain areas where access for sources is undesirable, impossible or not permitted. For example, a vibrator may not be able to cope with the topography or ground conditions (e.g., hills, dunes, wadis, sabkha, swamp), or access may be restricted for safety/security reasons (e.g., near industrial or military installations, on farms, in urban areas). In some of these cases it may still be possible or acceptable to deploy receivers in such areas. While advantageous, this still leaves a gap in the coverage at short to mid source-to-receiver offset ranges.
In some circumstances, such an “exclusion zone” or obstacle can be “undershot” from the outside. The exclusion zone could be populated with receivers, while sources can be deployed adjacent to the exclusion zone. Interferometry can then be used to construct virtual sources and/or virtual receivers, for which “virtual shot records” can be created for locations within the exclusion zone. This fills in part of the missing data, though with (virtual) sources only at the positions of the receivers (or, in some cases virtual receivers at positions of sources).
By deploying receivers at inaccessible source locations within the exclusion zone, it is possible to construct a partially virtual data set employing a source/receiver geometry as if no exclusion zone had existed.
Accordingly, there is a need for methods and systems that can employ faster, more efficient, and more accurate methods for surveys that include zones where sources and/or receivers cannot be deployed during the survey. Such methods and systems may complement or replace conventional methods and systems for surveying.